The present invention is concerned with secondary and tertiary recovery of petroleum (oil) from subterranean formations. In particular, the present invention is concerned with a system wherein water is the principal driving force of the recovery medium and more particularly, water systems containing partially hydrolyzed polyacrylamide polymers. Generally such systems may be classified as improved water flooding as compared for example, to secondary recovery using water alone, which is called "water flooding".
In the recovery of oil from underground reservoirs, even with the continued improvement in primary recovery techniques, substantial quantities of oil remain in the reservoirs. It has been estimated that only about 10 to 30% of the oil in the reservoir can be feasibly recovered by primary recovery techniques.
Due partially to its ready availability water has been extensively employed as the driving medium in secondary oil recovery. Although water flooding has been effective in obtaining additional oil from subterranean oil reservoirs, it has several deficiencies. Possibly, the most often noted of these is the tendency of flood water to "finger" through a reservoir and to bypass substantial portions of the reservoir. In other words, a water drive has less than perfect "sweep efficiency", in that it does not contact all portions of the reservoirs, and actual displacement is normally below its theoretical capacity.
"Fingering" is generally understood to result from the fact that the oil reservoirs possess regions and strata that have markedly varied permeabilities. The water, of course, flows more rapidly through those regions and strata having a greater relative permeability. As a result water flooding often misses substantial portions of the reservoir.
Early secondary flooding techniques employing a single fluid treatment are represented by the following U.S. Pat. Nos.: 1,651,311; 2,288,857; 2,341,500; 2,533,546; 2,731,414 and 2,771,138. Multiple injections have also been employed and represent an advance in the art, for example the injection of organic acid solutions followed by soap forming materials such as sodium hydroxide as exemplified by U.S. Pat. Nos. 3,174,542; 3,185,214; 3,111,984 and 3,368,621.
The use of a caustic solution as means to free the residual oil from the formation has long been practiced and is currently employed as shown by U.S. Pat. Nos. 1,651,311 (caustic added to a flooded reservoir); 3,175,610 (NaOH flooding); 3,414,053 (NaOH with soluble carbonate flooding) and 3,464,492 (ammonia introduced into formation to form soaps with acids in formation oil).
Various viscosifying agents such as natural or synthetic polymers are frequently employed in flooding solutions as shown in U.S. Pat. Nos. 3,039,529 (polymer); 3,372,748 (alternating slugs of polymer and caustic); 3,580,337 (polymer and divalent cations); 3,687,199 (two polymer slugs) and 3,825,067 (polymer and alkali).
Preinjection of polymers prior to a micellar slug (also called the surfactant or caustic slug) has been known and is examined by M. K. Dabbous in an article entitled "Displacement of Polymers In Waterflooding Porous Media and Its Effects on a Subsequent Micellar Flood", Society of Petroleum Engineers Journal, October 1977, pp. 358-368.
It can be readily seen that there has been an abundance of variations and combinations to raise the percentage of the residual formation oil which is recovered. It is a feature of the present invention that the particular combination of multiple treatment fluids provides a substantial increase in the oil recovery over either of the fluids alone or similar combinations of different fluids.